Removing manual beams when multiplying note values

[Nicholas Moe]

Hello,

I am replicating a handwritten score of Renaissance polyphony. I
entered the notes with \autoBeamOff and manually entered the beams
where they were needed using square brackets. Later, I decided I
wanted to double the values of the notes, as is common with this type
of music. I figured out how to do it by using a snippet in "The
LilyPond Report #19" and removing the beam stencils. But now I get all
sorts of errors. How do I suppress these?

Thanks,

Nick

%%%% BEGIN MINIMAL EXAMPLE %%%%
\version "2.13.61"

looksSlower =
#(define-music-function (parser location music) (ly:music?)
 (let ((new-music (ly:music-deep-copy music)))
   (shift-duration-log new-music -1 0)
   new-music))

\relative c' {
        \looksSlower{
                \autoBeamOff
                \override Beam #'stencil = ##f
                a8[ b]
        }
}
%%%% END MINIMAL EXAMPLE %%%%

%%%% OUTPUT EXCERPT %%%%
Interpreting music...
lilyPondTmp.ly:13:16: warning: stem does not fit in beam
                
                a8[ b]
lilyPondTmp.ly:13:18: warning: beam was started here
                a8
                  [ b]
lilyPondTmp.ly:13:20: warning: stem does not fit in beam
                a8[
                    b]
lilyPondTmp.ly:13:18: warning: beam was started here
                a8
                  [ b]
Preprocessing graphical objects...